This invention relates to apparatus for producing electric discharges in flowing gas. More particularly, it relates to apparatus for producing an arcless glow discharge in such gas. Such glow discharges are used in high power lasers and are also finding increasing use in materials surface treatment operations using plasmas.
In high power apparatus the flow of the gas mixture used therein permits the operation of a glow discharge at high input powers without excessive temperature increases in the apparatus. In high power flowing gas lasers the apparatus generally includes a closed loop fluid path with the heated gas from the glow discharge being cooled by passage through a heat exchanger prior to recirculation through the discharge. In all such devices the basic problem confronted is the provision, at high input power, of the desirable broad and uniform glow discharge from constricting into a small hot arc. Various prior art techniques have been used to help control this arcing tendency. The most common prior art technique has been the utilization of a large number of individual electrodes, each having its own ballast resistor, such as described in Bridges, U.S. Pat. No. 3,577,096, Foster et al, U.S. Pat. No. 3,772,610 and Eckbreth et al, U.S. Pat. No. 3,777,279. In these prior art structures individually ballasted multiple electrodes have been used for cathodes, for anodes and for both electrodes together. However, these prior art devices suffer from the deficiency that individually ballast multiple electrode structures are both complicated to fabricate and assemble and are also generally unreliable and short-lived because of this complexity.